Field of the Invention
The present invention relates to a process for the preparation of bulk or thin-film single-crystals of cubic sesquioxides (space group No. 206, Ia-3) of scandium, yttrium or rare earth metals doped or not doped with lanthanide ions having a valency of +III by a high-temperature flux growth technique and to the various applications of the single-crystals obtained according to this process, in particular in the optical field.
Description of Related Art
Pure cubic sesquioxides, that is to say nondoped cubic sesquioxides, are crystals which can be used as rotator crystal in Faraday isolators, which are composed of four constituents: an input polarizer, a rotator crystal, a permanent magnet and an output polarizer. This is because the development and the improvement in laser technologies have resulted in a need for optical components which protect the laser cavity from back reflections, Faraday isolators make it is possible to effectively suppress instabilities and fluctuations in intensity in laser systems. They can be used to protect the cavity in gas- or solid-state lasers, and also increasingly powerful laser diodes, from back reflections, as mentioned above, but also to prevent parasitic oscillations in multistage solid-state laser amplifiers. Faraday isolators are optical components which make it possible for light to move in just one direction. Their mode of operation is based on the Faraday effect, which effect proceeds in transmission and is linear in magnetic induction and in wavelength. For the majority of wavelengths, the rotator crystal is a terbium gallium garnet (TGG) placed in a high and homogenous magnetic field. It is in the latter that the polarization of the light ray rotates, according to a “Faraday” angle, proportionally to the Verdet constant, to the distance covered in the rotator crystal and to the electromagnetic induction. In general, these three parameters are adjusted so that the output polarization is rotated by 45°. If a light ray of any polarization comes in the opposite direction, then its polarization is rotated in the same direction: it is the nonreciprocal nature of the Faraday effect which makes it possible in particular to isolate very powerful laser cavities. However, the maximum isolation of the Faraday isolator is limited by the nonuniformities in the TGG crystal and the electromagnetic induction. There thus exists a need for single-crystal cubic sesquioxides which can advantageously be used as rotator crystal in a Faraday isolator, in particular which are capable of enduring high powers in continuous operation.
Solid-state lasers use solid media, such as crystals or glasses, as medium for the emission (spontaneous and stimulated) of photons and amplifier medium. The amplifier medium, or also gain medium, is composed of an optically active material comprising a matrix (glass or crystal) rendered optically active by doping with an ion which absorbs the radiation from an optical pumping source and which is de-excited by emission of photons. The first laser is a ruby laser, the emission of which originates from the Cr3+ ion. Other ions are much used: the majority are rare earth metal ions: Nd3+, Yb3+, Pr3+, Er3+, Tm3+, Eu3+, . . . , or also transition metal ions, such as Ti3+ or Cr3+, inter alia. The emission wavelength of the laser depends essentially on the doping ion for the rare earth metal ions and on the properties of the matrix in all cases, the influence of the latter being much greater in the case of the transition metal ions. Thus, glass doped with neodymium does not emit at the same wavelength (1053 nm) as the crystalline solid known as yttrium-aluminium-garnet (YAG) and composed of Y3Al5O12 doped with neodymium (1064 nm). Solid-state lasers operate in continuous mode or in pulsed mode (pulses from a few microseconds to a few femtoseconds). They are capable of emitting equally well in the visible region, the near infrared region, the middle infrared region and the ultraviolet region.
Above a crystal dimension of acceptable optical quality, these lasers make it possible to obtain powers of the order of approximately ten watts continuously and higher powers in pulsed mode. They are used for both scientific and industrial applications, such as welding, marking and cutting of materials.
In addition to their use in the manufacture of high-power lasers and/or short-pulse lasers, these solid materials, formed of a matrix and of a doping ion, can also be used in the manufacture of eye-safety lasers, of lasers for surgery and/or ophthalmology (diode-pumped lasers, pulsed or continuous, in the red region, the green region and up to the middle infrared region), of scintillators, of waveguides, of bolometers (detectors having heat/light discrimination), for optical cooling, as luminophoric materials or alternatively as materials for the storage and handling of quantum information.
At the current time, the most promising crystalline solids for all of these applications, and in particular for the manufacture of lasers, are cubic (thus isotropic) sesquioxides of formula R2O3 in which R represents one or more elements chosen from metals having a valency of III, such as scandium, yttrium and the rare earth metals (La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu), doped with rare earth metal ions. Some of them exhibit in particular a greater thermal conductivity than that of YAG doped with rare earth metal ion, which is nevertheless the most widely used laser material at the current time. These cubic sesquioxides are also advantageous insofar as they can be easily doped with rare earth metal ions and have a high density (of the order of 4 to 9.5 g·cm−3 approximately). Furthermore, yttrium, scandium, gadolinium and lutetium sesquioxides exhibit low phonon energies in comparison with the majority of oxides, in particular YAG.
These materials are mainly obtained in the form of transparent ceramics prepared by high pressure and high temperature sintering, preferably under vacuum. However, these ceramics exhibit a polycrystalline microstructure with numerous grain boundaries, diminishing the physical properties at the basis of their applications (diffusion of photons, presence of impurities, low thermal conductivity, limited degree of doping, and the like).
A process for the synthesis of single-crystal cubic sesquioxides targeted at overcoming the disadvantages of the processes previously known from the prior art has already been provided in the international application WO 2011/055075, The process provided was targeted in particular at obtaining, in a simple and inexpensive way, single-crystal cubic sesquioxides of scandium, yttrium or rare earth metal doped with rare earth metal ions exhibiting, with an equivalent chemical composition, a greater size than that of the single-crystals obtained according to the processes of the prior art, while having very good optical properties. The sesquioxides prepared according to the process described in the international application WO 2011/055075 correspond to the following formula R12O3:R2, in which R1 is at least one metal having a valency of III chosen from scandium, yttrium and the elements of the series of the lanthanides, that is to say from the elements having an atomic number ranging from 57 (lanthanum) to 71 (lutetium) according to the Periodic Table of the Elements, and R2 is at least one element chosen from the series of the lanthanides. This process consists in preparing a pulverulent mixture comprising at least one solute, composed of a mechanical mixture of at least one sesquioxide of formula (R′12O3)1−x in a molar percentage (1−x) and of at least one sesquioxide (R′22O3)x in a molar percentage (x) in which R′2 is identical to R1 and R′2 is identical to R2, and a synthesis solvent of following formula: [Li6(R″11−x′, R″2x′)(BO3)3], in which R″1 and R″2 are respectively identical to R1 and R2 and x′=x, in then bringing said pulverulent mixture to a temperature at least equal to the melting point of said mixture and ≤1250° C., in order to bring about the dissolution of the solute in the solvent of formula [Li6(R″11−x′, R″2x′)(BO3)3] and to obtain a liquid solution of said solute in said solvent, and in then carrying out the growth of the crystal on a solid support under controlled temperature conditions.
Although this process made it possible to overcome certain disadvantages brought about by the processes of the prior art, in particular in that it makes it possible to achieve crystals of centimetric size in their greatest length, it nevertheless still exhibits certain disadvantages, In particular, the crystallogenesis process is very slow (at least 40 days on average) and the crystals obtained nevertheless do not always have a sufficient size due to a very high polynucleation during the crystallization process, resulting in a reduction in the mean size of each single-crystal. Furthermore, the crystals obtained exhibit numerous solvent inclusions. These inclusions limit the working volume of the crystals, that is to say the volume where there are no inclusions, which can reduce the working volume to a size of 4×2 mm2 for a single-crystal initially having a centimetric size after synthesis.